The field of the invention is overload relays, and particularly, bimetal overload relays such as that disclosed in U.S. Pat. Nos. 3,267,236 and 3,852,694.
Bimetal overload relays include a snap action electrical switch which has a contact that is movable between an unactuated and an actuated position to make or break electrical connection with a stationary contact. This movable contact is mechanically coupled to a main bimetal element that is responsive to changes in temperature to operate the electrical switch. A heater unit is disposed adjacent to the main bimetal element and is connected in circuit with the device to be protected from current overloads. As the current flowing in this circuit rises, the temperature of the heater unit increases causing the main bimetal to change shape. When a predetermined current level is reached, the electrical switch is mechanically actuated, or tripped, by this motion of the bimetal element.
Such bimetal relays are of two general types with respect to the manner in which they are reset after being tripped by a current overload condition. In some, the snap action switch is biased so that as soon as the bimetal element cools, the switch returns to its unactuated position automatically. Other bimetal relays, however, have a manual resetting action in which a button, or plunger, must be operated to reset the snap action switch to its unactuated position. In installations where manually resettable type bimetal relays are employed, it is important that the reset mechanism not be easily defeatable by jamming the button or plunger in a depressed position so that the switch is automatically reset when the bimetal cools.